Chapter 14: Microphones and speakerssolutions

1.[2 marks](VCE 2004 Q1)

The horizontal scale is 1 division = 100s, and the number of divisions between cycles is 5.

The period is 5  100s = 5  10-4s.

Frequency =

=

= 2000Hz

= 2kHz (ANS)

2.[2 marks](VCE 2004 Sample Q1)

M1 is a dynamic (moving coil) microphone.

M2 is an electret-condenser microphone, as this is the only microphone that has a permanently charged backing plate.

3.[4 marks](VCE 2004 Sample Q2)

M1

The pressure changes on the coil move the coil in and out. This induces an EMF because the coil is in a magnetic field, and so there is a change in flux.

M2

The pressure changes on the diaphragm at the front of the microphone moves it in and out. This changes the space between the diaphragm and the charged backing plate. This changes the capacitance which leads to a voltage signal that leads to a current through the resistor, and hence a signal out.

4.[2 marks](VCE 2004 Sample Q3)

Reliably detected means that you require the truest replication from the microphone(fidelity). For fidelity of reproduction you need the flattest curve possible. A 3dB change from top to bottom will only just be audible to a human. Anything under this is not picked up by a human.

 the flattest region is from 100 to 2000

 C (ANS)

5.[2 marks](VCE 2004 Sample Q4)

The CRO will add the two sine curves. The combined signal must still be cyclic and both frequencies will be obvious.

 A (ANS)

6.[3 marks](VCE 2004 Pilot Q1)

These descriptions should be on your formula sheet.

Microphone type / Description
Dynamic (moving coil) / The sound pressure moves the cone which is attached to a coil of wire. This coil moves in the field of a magnet and produces a voltage.
Velocity (ribbon) / The sound pressure moves the metallic strip in the magnetic field, generating a voltage between the ends of the strip.
Electret-condensor (capacitor) / The sound pressure changes the spacing between a thin metallic membrane and the stationary back plate. This causes a change in capacitance and produces a current.

7.[2 marks](VCE 2004 Pilot Q4)

Speaker 2, because the graph is greatest in the 100 to 1000Hz range

 B (ANS)

8.[2 marks](VCE 2004 Pilot Q5)

Speaker 4, because the graph is amplified and flat in the 500 to 10 000Hz range

 D (ANS)

9.[2 marks](VCE 2004 Pilot Q6)

The sound waves from the back of the speaker can diffract around to the front and interfere destructively with the waves from the front. This is because they are out of phase.

The board prevents sound diffracting around the speaker and creating interference.

 the sound is louder.

10.[4 marks](VCE 2004 Pilot Q7)

The high cut-off for speaker 3 is about the same as the low cut-off for speaker 4 and hence they will complement each other.

They also have about the same output over this range.

 Speakers 3 and 4 (ANS)

11.[3 marks](VCE 2005 Q2)

These descriptions should be on your formula sheet.

Microphone type / Principle of operation
dynamic / electromagnetic induction
crystal / piezoelectric effect
Electret-condenser / capacitance

12.[2 marks](VCE 2005 Q3)

The role of the baffle board, (the enclosure) is to minimise the interaction (interference) between the wave produced at the front of the speaker and the wave produced at the rear of the speaker. Sound from the back and front of the speaker are out of phase. When a compression is produced at the front of a speaker a rarefaction is produced at the back. If this rarefaction was to diffract and travel into the space in front, then partial cancellation would be possible. The baffle board prevents the sound from the back of the speaker cones cancelling the sound from the front.

13.[2 marks](VCE 2005 Q4)

The best response was in the high frequency range. Above 20 000Hz, is not that important, because very few people can actually hear these frequencies.

 D (ANS)

14.[3 marks](VCE 2005 Q5)

A loudspeaker is omni-directional, (i.e. it radiates sound energy spherically in all directions). The amount of diffraction (spreading) depends on the ratio.

For a speaker of fixed size (w), the higher the frequency (shorter wavelength) the less the diffraction away from the centre line. Hence the fidelity will deteriorate as the listener moves off the centre line. It would be more satisfactory if the high frequency sound is from a loud-speaker with a smaller diameter (a tweeter).

Using different speaker sizes for different frequency ranges would ensure that comparable spreading will occur across all frequencies.

15.[2 marks](VCE 2006 Q6)

The role of the baffle board, (the box) is to minimise the interaction (interference) between the wave produced at the front of the speaker and the wave produced at the rear of the speaker.

Sound from the back and front of the speaker are out of phase. When a compression is produced at the front of a speaker a rarefaction is produced at the back. If this rarefaction was to diffract and travel into the space in front, then destructive interference would be possible.

16.[2 marks](VCE 2006 Q10)

C (ANS)

This definition should be on your formula sheet.

17.[2 marks](VCE 2006 Q11)

The sound pressure moves the cone which is attached to a coil of wire. This coil moves in the field of a magnet (changing the flux) and a voltage is induced.

18.[3 marks](VCE 2007 Q1)

A loudspeaker is removed from its enclosure box. When an audio signal is connected,the loudspeaker produces sound waves at both its front and rear surfaces. The sound wavesfrom the front of the loudspeaker are[out of phase with] the waves from therear. For a listener in front of the speaker the waves from the front[interfere destructively] with thosegenerated from the rear surface. This affects the [intensity]of the resulting sound.

19.[2 marks](VCE 2007 Q6)

The sound pressure moves the cone which is attached to a coil of wire. This coil moves in the field of a magnet (changing the flux) and a voltage is induced.

20.[2 marks](VCE 2007 Q7)

The flat response gives the truest replication from the microphone(fidelity). For fidelity of reproduction you need the flattest curve possible. The curve is very flat between 200Hz and 3000Hz. This region corresponds to the majority range of the human voice.

21.[2 marks](VCE 2008 Q1)

The physical property of the electret-condenser microphone is capacitance

The physical property of the crystal microphone is the piezo-electric effect

The physical property of the dynamic microphone is electromagnetic induction.